The monoplastic Protophyta—that is, those primary Algæ formed by a single plastid—are of the greatest interest, because the vegetable organism in this case completes its whole course of life as a perfectly simple “individual of the first order,” either as a cytod without kernel, or as a cell containing a kernel.
Among the primary plants consisting of a single cytod are the exceedingly remarkable Siphoneæ, which are of considerable size, and strangely “mimic” the forms of higher plants. Many of the Siphoneæ attain a size of several feet, and resemble an elegant moss (Bryopsis), or in some cases a perfect flowering plant with stalks, roots, and leaves (Caulerpa) (Fig. 17). Yet the whole of this large body, externally so variously differentiated, consists internally of an entirely simple sack, possessing the negative characters of a simple cytod.
Fig. 17.—Caulerpa denticulata, a monoplastic Siphonean of the natural size. The entire branching primary plant, which appears to consist of a creeping stalk with fibrous roots and indented leaves, is in reality only a single plastid, and moreover a cytod (without a kernel), not even attaining the grade of a cell with nucleus.
These curious Siphoneæ, Vaucheriæ, and Caulerpæ show us to how great a degree of elaboration a single cytod, although a most simple individual of the first order, can develop by continuous adaptation to the relations of the outer world. Even the single-celled primary plants—which are distinguished from the monocytods by possessing a kernel—develop into a great variety of exquisite forms by adaptation; this is the case especially with the beautiful Desmidiaceæ, of which a species of Euastrum is represented in Fig. 18 as a specimen.
Fig. 18.—Euastrum rota, a single-celled Desmid, much enlarged. The whole of the star-shaped body of this primæval plant has the formal value of a simple cell. In its centre lies the kernel, and within this the kernel corpuscle, or speck.